Repeater systems for radios are well known in the art. Repeaters are normally radios which are fixed in a single geographical location, with a favorably situated antenna that can provide communication services to a large geographical area. The communication services are typically provided to vehicular mobile radios and/or in-hand portable radios. Repeaters are often classified as fixed equipment, and the attached radio system is often referred to as fixed infrastructure equipment. The mobile and/or portable radios are often referred to generally as subscriber units. Fixed repeater equipment is advantageous for covering the largest possible area, but it often cannot provide coverage in areas that are obstructed relative to the fixed antenna location. This condition is particularly common when the subscriber units in a system have small antennas and when the subscriber units are operated inside of buildings or behind obstructions.
An especially useful type of fixed infrastructure equipment is for trunked radio systems. A trunked fixed infrastructure typically uses several radio frequency (RF) channels as communication resources. These resources are assigned dynamically to subscriber units as they demand service, and the resources are released for other subscriber units after the demanded service is completed. The subscriber units demand services by transmitting service requests to the trunking fixed infrastructure via a special resource designated as a control channel. The assigned communication resources are generally known as traffic channels. Traffic channels can be used to transmit voice information, data information, or other useful information that users of subscriber units want or need. The communication resources can be provided by Frequency Division Multiple Access (FDMA) radio channels, by time slots on a Time Division Multiple Access (TDMA) radio channel or channels, by spreading codes on a Code Division Multiple Access (CDMA) radio channel or channels, or any other feasible multiple access technology.
Mobile repeater systems for extending the coverage of fixed equipment are well known in the art. FIG. 1 illustrates a prior art embodiment of a wireless communication system 100 comprising a fixed infrastructure 101, a subscriber unit 102 and mobile repeater 103. The fixed infrastructure 101 typically consists of fixed repeaters, antennas, and other equipment which provides coverage throughout a large geographical area. Although shown as a single element in FIG. 1, the fixed infrastructure 101 typically encompasses multiple repeaters at multiple sites, each with one or more antennas, and any other equipment necessary to interconnect and/or control such multiple sites and repeaters. The subscriber unit 102 obtains service from the fixed infrastructure 101 via a traffic channel 106 and a control channel 107. The control channel 107 is used by the subscriber unit 102 to request service, and the fixed infrastructure 101 typically provides service by assigning a traffic channel 106. Often, the traffic channel 106 conveys voice communications, but it can also convey data or other useful information for the subscriber unit 102. The subscriber unit 102 communicates directly with other subscriber units (not shown), or via communication resources provided by the fixed infrastructure, such as telephone service via telephone wire lines. The mobile repeater 103 can act as a subscriber unit via the same traffic channel 106 or control channel 107. The mobile repeater 103 can extend coverage of a radio system by repeating the traffic channel 106 on an extended channel 108 which can be used by the subscriber unit 102 if it is not in range of the fixed infrastructure 101.
In FIG. 1, the extended channel 108 does not extend the control channel 107 to the subscriber unit. As such, the control information to request service from the fixed infrastructure 101 or to deliver service from the fixed infrastructure cannot extend to the subscriber unit 102 over the extended channel 108. Only the basic bearer service itself, such as the voice information, can be extended. This means that control information, such as a unit identification (ID) or a talk group ID, has to be synthesized by the mobile repeater 103 without any connection or assistance from the subscriber unit 102. This also means that the mobile repeater 103 is unable to determine if the subscriber unit 102 is in range to receive any delivered service, nor is the mobile repeater able to deliver useful services like call alerts, pages, or other control information. In essence, the extended channel 108 reduces the range of available service to the subscriber unit, when compared to the service provided by the fixed infrastructure 101 via the control channel 107 and traffic channel 106.
The problem of extending a control channel has been addressed in U.S. Pat. No. 5,179,720 by Grube et al. entitled METHOD AND APPARATUS FOR EXTENDING COVERAGE OF A TRUNKED RADIO COMMUNICATIONS SYSTEM (hereinafter "the '720 patent"). The '720 patent explains how to repeat an infrastructure-based control channel so that a subscriber unit is provided with a duplicate control channel. The assignment of communication resources is a problem in such a system because the extended resources are not the same as the resources controlled by the infrastructure. Consequently, if channel assignments from the fixed infrastructure are simply repeated without conversion, it would appear to the otherwise out-of-range subscriber unit that it has been assigned infrastructure-based resources instead of extended resources, and the subscriber unit would not be able to communicate. The '720 patent solves this problem by providing repeaters that map the infrastructure-based resources to extended resources, and vice versa, so that communications may take place. In this way, the simplest trunking fixed infrastructure service can be extended through a control channel to the subscriber unit.
While the '720 patent explains how to map infrastructure-based resources to extended resources using either a fixed or mobile repeater, it fails to describe how to provide other trunking services. In particular, subscriber units are mobile enough to move between the coverage areas of different repeaters, or from the coverage area of a fixed infrastructure to that of a mobile repeater. When the subscriber unit moves to a new coverage area, it needs to notify the fixed infrastructure or the mobile repeater of this change so that service can be properly routed to the subscriber unit. Again, referring to FIG. 1, if the subscriber unit 102 starts in the coverage area of the fixed infrastructure 101, it begins by registering with the fixed infrastructure 101. This allows the fixed infrastructure 101 to know to assign communication resources 106 to the subscriber unit 102 when service is necessary. If the subscriber unit 102 goes out of the coverage area of the fixed infrastructure 101 and into the coverage area of the mobile repeater 103, then it needs to register its presence with mobile repeater 103. This would allow the mobile repeater 103 to assign resources 108 for any service to/from the subscriber unit 102. The '720 patent does not explain how this could be done.
Additionally, the prior art does not explain how to protect the communication resources for the system, namely how to encrypt them. The protection of communication resources from unauthorized or undesired use is typically accomplished by encrypting the infrastructure-based control channel(s). This encryption is done in a synchronous manner so that the control channel(s) can be decrypted by subscriber units, the fixed infrastructure, or mobile repeaters. However, the prior art fails to teach how to extend encrypted control channels to subscriber units.
In view of these shortcomings of the prior art, it would be advantageous to provide a mobile repeater that can extend coverage of trunked radio systems so as to provide full registration and protection of the services of the system.